1. Field of the Invention
This invention relates to recuperative heat exchangers of the formed plate type and, in particular, to plate structures adapted to transfer heat from one fluid to another through the surface of the plate and to the methods of forming such.
2. Description of the Prior Art
Recuperative heat exchangers are known in which a plurality of plates of relatively thin material are formed and stacked so as to provide heat transfer through the plates to and from a series of alternate flow passages formed between alternate pairs of plates.
In the interchange of heat between the fluid passages and the heat exchanger, fluids are separated by a plate of high thermal conductivity. In order to obtain the maximum efficiency, the design of the heat exchanger must take into consideration several critical factors. Among these factors which affect the efficiency of design are: (1) the amount of heat transfer area in intimate contact with the fluid, (2) a boundary layer resistance of the plate to the exchange of heat between the fluids. (3) the difference in thermal conductivity of the various parts of the heat exchanger, and (4) the overall structure of the heat exchanger core. The design of prior art heat exchangers has resulted in compromises in design according to the above factors whereby fluid capacity suffers with a decrease in size or vice versa, because of the inability of designing a heat exchanger to optimize all of these factors. For example, strength and reliability of the overall structure dictate some parts of larger size than others resulting in large differences in thermal conductivity of the parts. Conversely, if the parts are of the same size the overall structure may be too weak to stand the pressure and temperature gradients therein or may be too heavy and cumbersome for practical use in any but the most limited applications.
Various attempts have been made to solve the above-noted problems in heat exchangers by designing plate type heat exchangers comprising a series of stacked thin metallic plates which are assembled in face-to-face arrangement to define fluid passages therebetween for separate flow of primary and secondary fluids in the heat exchange relation. A thin plate type of heat exchanger has been generally very difficult to manufacture, due to the many welding and bonding operations required, and difficult to achieve a strong structure because of the thinness of the plate material.
A preferred type of heat exchanger would have a uniform thickness of plate and other components utilized throughout the heat exchanger in order to maintain a uniform thermal conductivity between the parts. In this manner localized adverse expansion and contraction effects encountered during the heating and cooling cycle would be minimized. In the interest of maintaining a low cost of manufacture it would be highly advantageous to make a heat exchanger of plates which are similar in structure and form so as to present surfaces adapted to mate with each other to contribute adequate seals. In prior art heat exchangers typically a module of two plates is provided, wherein the plates are recessed to accommodate the flow of two fluids so as to provide a reliable and effective basis for sealing around the module perimeter as well as adequate structural strength. However, to accomplish this the sealing is generally provided by bars which are welded or brazed to the stacked plates. The great difference in thermal conductivity of the bars, as compared to the thin material of the plates, has a deleterious effect on the heat exchanger, causing undesirable stresses during expansion and contraction of the stacked parts. Thus, the plate type heat exchanger of the prior art, which is formed of a series of plates stacked together in spaced side-by-side relation, has been limited in efficiency due to the above-mentioned disadvantages therein.
Accordingly, it is essential that the heat exchanger be designed with the above-mentioned factors taken into consideration in order to achieve a low cost, high efficiency heat exchanger. In a typical prior art recuperative heat exchanger, the type of construction is usually characterized by a large number of components which result in high labor efforts with resulting high cost of fabrication of the heat exchangers. Structural problems are associated with the thermal inertia incompatibility of the different size core elements and these severely limit the design objective. Existing heat exchangers for large industrial gas turbines realize a fairly low compactness resulting in a unit of extremely large volume and weight. On the other hand, providing a heat exchanger of high compactness results in an extremely high cost of manufacture. Prior attempts at providing a more compact heat exchanger of low cost and high efficiency have met with failure due to the inability to solve the problems set forth above.
The design of a plate type heat exchanger must take into consideration the transient metal temperature differentials between the various parts. These differentials occur during thermal transients and are caused by the temperature time lag of the relatively heavier sections in the core, such as the bars which may be used to enclose the relatively thin stacked plates. These heavier reinforced bars for sealing the gauge manifolds are thermally incompatible with the plates. Additionally, the heavy gauge manifolds which are required for the input and output fluid passages often result in a transient thermal stress at the ends of the core matrix which exceeds the material yield strength. Of course, if the manifold sections were designed of thin materials, the structural strength of the heat exchanger core would be unacceptable.
Thus, it may be seen that a heat exchanger is desired that will achieve the thermal inertia compatibility between the various elements of the core without sacrificing the structural strength and efficiency of the heat exchanger. Such a structure should desirably admit of fabrication without inordinate labor costs to be commercially feasible.